Gas sensors of the kind described above exist in various different embodiments.
These kinds of gas sensors have a cavity or space that functions as a gas cell through which rays of light are allowed to pass. Interacting with the gas cell are light emitting means and light receiving means (light receiver). The light receiver is designed to enable an evaluation of the current lines of absorption in the light spectrum for an exiting beam of light, or rays of light.
Within the cavity, between the light emitting means and the light receiver in the gas cell, is a light path, hereafter called a measuring path or optical measuring path.
In optical applications the terms "geometrical path" and "optical path" are sometimes used, where the geometrical path is a geometrical distance and the optical path is a geometrical distance multiplied with the refractive index of the medium through which the light passes. In so called "standard air", where the refractive index is 1, the optical path is thus equal to the geometrical path.
The terms "measuring path" and "optical measuring path" are used synonymously in this description since the gas concentrations at hand are very low which gives a refractive index very close to 1.
Even if the refractive index would deviate substantially from 1, for example at a measurement of a fluid, these terms could be used synonymously since the used technique is based on reflections of light in a homogeneous medium and not on transmissions of light through various mediums with different refractive indexes, wherefore the compensations for variations in the refractive index is not required. It is nevertheless obvious for a person skilled in the art what measures are required if the light emitting means and/or the light receiving means are positioned in an optical medium that differs in refractive index from the medium inside the gas cell.
It is also known that thorough analysis of gas type, gas mixture, and/or gas concentration in a gas sensor with a gas cell is based on the following relationship: when concentrations are low, and when the absorption spectrum for a given gas is not expressly obvious, then a long measuring path is required for a light beam that passes through a gas sample in order to achieve an accurate result.
Today, a measuring path of approximately 0.1 meters is required in most practical applications that use current technology to produce beams of light, to detect and receive beams of light after they have been reflected a given number of times, and to analyse the absorption spectrum for the beam of light.
The U.S. Pat. No. 5,163,332 shows and describes a gas sensor that can be used to analyse gases. The gas sensor consists of a long, hole-shaped pipe with internal light reflective surfaces that allow the pipe to function as a channel or conductor of light, thereby creating a measuring path for transferring beams of light from a light source to a defector, and through a gas sample enclosed in the pipe.
The beams from the light source are arranged to be indirectly reflected by opposing surfaces--that is, by indirect reflection in all directions.
By reflection in all directions in an optical conductor, we mean that if rays of light, coordinated into a beam of light with diverging beams are allowed to enter a pipe whose inside has light-reflective properties, then the beams of light that angle away from the centre line, the z-axis, will be reflected in the x-z plane as well as in the y-z plane.
Further, several openings in the wall section of the pipe are shown equipped with filters, which permit a gas sample to freely be introduced into the pipe, or to freely exit from inside it.
Attempts have been made to reduce the external dimensions of a gas sensor or gas cell while obtaining a long measuring path.
The following publications are referred to as examples of the background art.
The U.S. Pat. No. 5,340,986 showed a diffusion type gas sensor, where the required length for the gas cell can be reduced by half, relative to a desired measuring path, by arranging a transmitter and a receiver in one end of the pipe, and a mirror in the other end of the pipe, as well as by giving the inside of the pipe light-reflective properties.
This method offers a directly-reflected light beam and indirectly-reflected rays of light.
The U.S. Pat. No. 5,060,508 made known a gas cell with an extended measuring path--relative to its external dimensions--shaped in a block with small external dimensions. The block is equipped with several canal sections, oriented at the front and back, and connected to one another. The walls for these connected canal sections are coated with a highly-reflective agent, causing the resultant passage to serve as an optical conductor, in order to transfer beams of light by means of indirect reflection in all directions. Several minor passages permit the gas in an area surrounding the pipe to diffuse into the passage.
In this example, the gas cell is created by positioning the two block halves against one another. These halves can be cast of plastic, making them relatively inexpensive to produce.
The content of the publication EP-A1-0 647 845 is also a part of the background art in this regard.
Considering the characteristics exhibited in the present invention, we should also mention that an absorption cell was previously made known by J. U. White, in the Journal of Optical Society of America, Vol. 32, page 285 (1942). A cell was shown and described, consisting of three spherically concave mirrors, each of which had the same radius of curvature, and was positioned to create a desired optical measuring path.
The cell was developed in order to obtain an extended measuring path for a directly-reflected beam of light in a gas sensor. By applying the principles described in the above-mentioned publication, it has been possible to develop gas sensors whose optical measuring paths exceed 10 meters in length.
Through this example, it is known to position the mirrors far apart, ordinarily 0.3 to 3 meters, and adjust the rays of light so that their diverging angle is very slight.
Further, the rays of light produced must not be interfered with by indirect collateral reflections. Instead, they must be reflected directly between the mirrors.
The U.S. Pat. No. 4,756,622 describes how other measures have been taken to create a long measuring path for wave length absorption in gas. For example, this publication explains that the light beam may wander through a limited volume of gas very many times; for example in the order of thousands of times.
The light beam is introduced to a closed optical loop, where it is allowed to circulate through the gas sample. Then, after having circulated through the gas sample a predetermined number of times, the light is deflected from the closed optical loop by means of polarising wave conductors.
Even when the reflective factor is as high as 0.998, after 100 reflections the intensity of the light is only 80% of its original brightness. After 300 reflections, it is only approximately 50% of its original brightness.